Dial-back call completion method and apparatus in a radio-communication system

ABSTRACT

A method and apparatus for making a call connection in a communication system (10) having multiple communication units (26) (CU), and at least one communication antenna (12) that moves with respect to the CUs (26). The method involves determining (154) whether a call attempt from a source CU (26) to a target CU (50) is successful. When the call attempt is not successful, a control facility (24) predicts (xxx) when one of the antenna (12) will move within an optimal call initiation area (124), within which there is an increased likelihood of establishing a communication link with the target CU (50). At the time when the antenna (12) will move within the optimal call initiation area (124), the system attempts to contact the target CU (50) and, if successful, informs the target CU (50) to wait while the system calls the source CU (26) and connects the call.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent application Ser.No. 08/519,221 entitled "Position-Dependent Call Connection Method andApparatus in a Radio-Communication System", by Daniel Richard Tayloe(Docket No. IRI03242), filed concurrently with this application andassigned to the same assignee as the present application.

FIELD OF THE INVENTION

The present invention relates generally to call connection in aradio-communication system, and more particularly to call connection ina satellite cellular communication system.

BACKGROUND OF THE INVENTION

In ground-based cellular communication systems, link margins of up to 35decibels (dB) may be readily achieved. These link margins allowsubscriber units to establish communication links where physicalobstructions exist between the subscriber unit and the cellular antenna.Thus, a person placing a call to a subscriber unit often maysuccessfully call a subscriber unit located inside a car, a building, oran urban canyon (i.e., an area surrounded by high-rises).

Although a ground-based system may have a relatively good link margin,the success rate for placing a call is often 30% or less. Low callsuccess rates result from excessive physical obstructions, widespreaduse of portable phones by other users, and spotty coverage by thecellular antennas. Generally, a person must make several call attemptsbefore establishing a link with a subscriber unit. At times of very highcommunication traffic, many attempts may be necessary, thus frustratingsystem users.

Often times, a subscriber unit that is used to place a call (i.e.,the"source unit") will be moved by the user to an area where little orno obstructions are located. This enhances the user's chance ofestablishing a link. However, the recipient of the call (i.e.,the"target unit") may be another subscriber unit that is located in anarea of excessive obstructions. The communication system may beincapable of supporting a link between the target unit and a cellularantenna.

Prior-art systems seek to alleviate these problems by boosting linkmargins, providing more channels, and increasing antenna coverage.However, the prior-art solutions are limited by the system's economicand physical constraints. Once a system is in place, prior-art systemsoften must settle for having a user make repeated attempts to establisha call.

A satellite communication system provides additional challenges to callestablishment because such a system is likely to have a significantlylower link margin (e.g., in the range of 15 dB or less). The lower linkmargin results from the satellite antenna being located at a muchgreater distance from the subscriber units. Of course, more powerfulequipment may provide a higher link margin, but such equipment is heavyand thus undesirable for a satellite application.

Additionally, for all satellite orbits except geosynchronous orbits,satellites move with respect to the subscriber units. Because of thelower link margins, a subscriber unit located in an urban canyon mighthave a small likelihood of establishing a link with a satellite that islow on the horizon. However, a satellite at a high angle of elevationmay have a greater likelihood.

The prior-art repeated attempt method of establishing a communicationlink is undesirable for such a satellite communication system because asource unit generally has no knowledge of satellite positions withrespect to a target unit. Thus, an unlucky user may try at various timesof an entire day while never being able to establish a call.

What are needed are a method and apparatus for providing a highlikelihood of establishing a call in a satellite communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a satellite communication system in accordance with apreferred embodiment of the present invention;

FIG. 2 illustrates a cellular projection from a satellite in accordancewith a preferred embodiment of the present invention;

FIG. 3 illustrates geometric communication limitations for a satellitecommunication system user in accordance with a preferred embodiment ofthe present invention;

FIG. 4 illustrates a possible call initiation area in accordance with apreferred embodiment of the present invention;

FIG. 5 illustrates optimal call initiation areas in accordance with apreferred embodiment of the present invention;

FIG. 6 illustrates a method for dial-back call completion at an optimalcall attempt time in accordance with a preferred embodiment of thepresent invention;

FIG. 7 illustrates a method for connecting communication units inaccordance with a preferred embodiment of the present invention;

FIG. 8 illustrates a method for calculating optimal call attempt timesin accordance with a preferred embodiment of the present invention;

FIG. 9 illustrates a communication unit in accordance with a preferredembodiment of the present invention; and

FIG. 10 illustrates a control facility in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention provides a method and apparatus for providing ahigh likelihood of establishing a call in a satellite communicationsystem. In summary, the method and apparatus of the present inventioncalculates opportunity times when there is a high likelihood ofestablishing a link between a target communication unit and a satellite.These opportunities are calculated by predicting the satellite's futurepositions with respect to an optimal call initiation area. The systemthen contacts the target communication unit at a future opportunity. Ifthe contact is successful, the system informs the target communicationunit to hold, contacts the source communication unit, and connects thesource and target communication units.

FIG. 1 illustrates a satellite communication system 10 in accordancewith a preferred embodiment of the present invention. Communicationsystem 10 includes control facility 24, orbiting satellites 12, andcommunication unit 26. Satellites 12 occupy orbits that can be low-earthorbits, medium-earth orbits, or a combination thereof. In a preferredembodiment, communications system 10 uses six polar orbital planes, witheach orbital plane holding eleven satellites 12 for a total of sixty-sixsatellites 12. However, sixty-six satellites 12 are not essential, andmore or fewer satellites 12, or more or fewer orbital planes can beused. For clarity, FIG. 1 illustrates only two satellites 12. However,only one satellite 12 is necessary to carry out the method of thepresent invention.

Satellites 12 communicate with terrestrial equipment which can be anynumber of communication units 26, control facility 24, other satellites12, and possibly other communication devices (not shown). Communicationlink 16 between satellite 12 and communication unit 26 is accomplishedby using communication cells projected by the satellites 12 toward theearth.

FIG. 2 illustrates a cellular projection from a satellite 12 inaccordance with a preferred embodiment of the present invention.Satellite 12 has communication antenna 30 which projects a footprint 28of communication cells 34 toward the face of the earth. Eachcommunication cell 34 can contain multiple channels which CUs may use.

Referring again to FIG. 1, communication link 20 between satellite 12and control facility 24 is accomplished by using a directradio-frequency or optical link. Communication link 22 betweensatellites 12 are referred to herein as"cross-links" and may not berequired where a single satellite 12 is geographically capable ofcommunicating with both a communication unit 26 and a control facility24.

A communication unit 26 (CU) can be, for example, a hand-held, portablecellular telephone adapted to transmit data to and/or receive data fromsatellites 12. A CU 26 can also be a facsimile device, pager, dataterminal, a telephone connected to a public switched telephone network(PSTN), or any other type of communication device. As used herein,a"source CU" is a CU that initiates an attempt to establish acommunication path with a"target CU". A communication path can includeany number of satellite-to-ground and satellite-to-satellite hops,depending on the location of the source CU relative to the target CU. Asused herein, a"target link" is the link along the communication pathbetween a satellite and the target CU.

A control facility 24 (CF) is an equipment facility, typicallyground-based, that is capable of interfacing with satellites 12. CF 24may communicate directly with satellites 12, or may communicate viaremote or co-located ground stations (not shown).

Multiple CFs 24 can exist within communication system 10, each capableof providing different functions. Some CFs 24 connect with ground-basedequipment such as, for example, a public switched telephone network(PSTN) (not shown). The same or other CFs 30 control operations ofcommunication system 10.

In a preferred embodiment, CF 24 determines when a source CU 26 hasfailed an attempt at a call due to the target CU's 26 inability toestablish a link with a satellite 12. CF 24 then calculates future callinitiation opportunities during which target CU 26 will have a higherlikelihood of link establishment. The communication system then attemptsto contact the target CU 26 during a future call opportunity. If thecommunication system is successful, the target CU 26 is asked to hold,the source CU 26 is contacted, and the connection is established. Themethod is described in detail in conjunction with FIGS. 6-8.

Only one of CF 24 and CU 26 are shown in FIG. 1 for clarity and ease ofunderstanding. In alternate embodiments, additional system facilitiesmay be desirable, depending upon the requirements of the communicationsystem.

FIG. 3 illustrates geometric communication limitations for a satellitecommunication system user in accordance with a preferred embodiment ofthe present invention. A CU 50 may be located in an area havingsignificant radio-frequency obstructions (e.g., buildings 70).Satellites 60-62 traveling along path 64 are required to support links72, 74 with the CU 50. Where obstructions 70 exist between satellites60, 62 and CU 50, links 72 may not be possible. However, where noobstructions exist between satellite 61 and CU 50, link 74 is highlypossible.

Angle 80 represents a minimum angle of elevation above the horizon atwhich communications are possible between a satellite 60-62 and a CU 50.A user in an area with little or no obstruction is likely to be able toestablish a link once a satellite 60-62 is located above angle 80.However, as illustrated in FIG. 3, a user in an urban canyon may not beable to establish a link until a satellite 60-62 has risen above theobstructions 70. Angle 85 represents an angle of elevation that asatellite must be located above for the CU 50 to establishcommunications. Angle 90 spans the area within which link 74 may bemaintained. The area defined by angle 90 is referred to herein as a"callinitiation area".

FIG. 4 illustrates a possible call opportunity area 100 in accordancewith a preferred embodiment of the present invention. Point 96represents a location of a CU. Call opportunity area 100 represents anarea defined by an angle of elevation that is high enough that there isa high likelihood that CU 96 will be capable of establishing a link witha satellite traveling in call opportunity area 100. Paths 102-107represent possible paths of a satellite with respect to call opportunityarea 100. Paths 102 and 107 fall outside call opportunity area 100.Thus, communications with satellites traveling along these paths isprobably not highly likely. Paths 103-106 fall within call opportunityarea 100 and communication with satellites traveling along these pathsis highly likely.

As FIG. 4 illustrates, some paths have longer intersections of callopportunity area 100. For example, a satellite traveling along path 104would be within call opportunity area 100 for a longer period of timethan path 103. Thus, a satellite traveling along path 104 would be ableto sustain a call for a longer period of time.

The length of time of a call also depends upon when a call is initiatedduring a satellite's path across call opportunity area 100. In apreferred embodiment, a source CU would want to initiate a call to atarget CU 96 when a satellite first enters call opportunity area 100.

FIG. 5 illustrates optimal call initiation areas 124, 130 in accordancewith a preferred embodiment of the present invention. CU 120 is locatedat the center of call opportunity area 122. Although satellites cantravel in any direction, FIG. 5 shows satellite 128 traveling in anorthbound direction and satellite 130 traveling in a southbounddirection.

Optimal call initiation area 124 represents the portion of callopportunity area 122 within which satellite 128 should initiate a callto provide the CUs with a minimum acceptable call duration. Likewise,call initiation area 126 represents the portion of call opportunity area122 within which satellite 130 should initiate a call. Areas 134 areexcluded because neither satellite 128 nor 130 would intersect callopportunity area 122 for a long enough period of time to provide anacceptable call duration. Additionally, area 136 is excluded because acall is optimally initiated soon after a satellite enters callopportunity area 122. In alternate embodiments, excluded areas 134, 136may be larger, smaller, or nonexistent depending on the duration ofcalls a communication system is willing to accept

Note that the relative locations of optimal call initiation areas andexcluded areas depends on the direction of travel of the satellites. Forexample, a satellite having a smaller angle of inclination from thehorizon may intersect call opportunity area 122 in a substantiallyeasterly direction. In such a case, an optimal call initiation areawould be located along the western portion of call opportunity area 122.

FIG. 6 illustrates a method for dial-back call completion at an optimalcall attempt time in accordance with a preferred embodiment of thepresent invention. The method begins in step 150 when a call attempt ismade in step 152. In step 154, a determination is made whether thetarget link is successfully established. This determination can be madeby a satellite or by a control facility. If the target link issuccessfully established, the method ends 168.

If the target link is not successfully established, the methodcontinues. In a preferred embodiment, the satellite or control facilityprovides the source CU with an option in step 156 of whether the sourceCU wants the system to calculate future call opportunities and providethe dial-back feature. In an alternate embodiment, the systemautomatically provides the dial-back feature. The source CU can providethe option via a display to the user of the source CU who can eitherexercise the option or not by entering her choice using an input device.If the user of the source CU indicates that she does not want the systemto provide the dial-back feature, the method ends 168.

The system can provide the dial-back feature as a special feature of thesystem by providing this option. If the user of the source CU decides toexercise the option, the system can charge the user a fee.

If the source CU indicates that it would like the system to calculatethe future call opportunities and provide the dial-back feature, the CFor satellite requests the phone number of the source CU in step 158. Thesource CU sends the phone number to the CF or satellite in step 160.Steps 158 and 160 need not be performed if the CF or the satellitealready has the phone number of the source CU.

Next, step 162 provides the dial-back feature and connects the source CUand the target CU. Step 162 is described in detail in conjunction withFIGS. 7 and 8. The method ends in step 168.

FIG. 7 illustrates a method for connecting communication units inaccordance with a preferred embodiment of the present invention. Themethod begins in step 180 by performing the step 182 of calculatingfuture call opportunities. For example, a future call opportunity can berepresented by the times a satellite (e.g., satellite 128, FIG. 5)enters and exits an optimal call initiation area (e.g., area 124). Thesystem may calculate only one opportunity or multiple opportunities. Thenumber of opportunities calculated may be requested by the source CU. Ina preferred embodiment, the calculations are performed in a controlfacility (e.g., CF 24, FIG. 1 ). In alternate embodiments, thecalculations are performed in a satellite or a CU. FIG. 8 describes thestep 182 of calculating future call opportunities in detail.

In a preferred embodiment, the future call opportunities are sent to thesource CU in step 184. This step is not required, but is desirablebecause it gives source CU knowledge of when in the future source CU maybe contacted by the system.

A determination is made in step 186 whether a next future callopportunity time has been reached. If not, the method iterates as shownin FIG. 7. If a next future call opportunity has been reached, thesystem attempts to contact the target CU in step 188. The system shouldhave a high likelihood of success because a satellite is within theoptimal call initiation area during the call opportunity time.

A determination is made in step 190 whether the system has successfullycontacted the target CU. If not, steps 186-190 are repeated for alimited number of attempts. The number of attempts made by the system isarbitrary. If contact is successful, the system informs the target CU tohold in step 192. While the target CU is holding, the system contactsthe source CU in step 194. After the system contacts the source CU, thesystem connects the source CU and the target CU in step 196. The methodends in step 198.

FIG. 8 illustrates a method for calculating future call opportunities inaccordance with a preferred embodiment of the present invention. Themethod begins in step 200 by performing the step 202 of determining alocation of the target CU. In a preferred embodiment, the location is alast-known location of the target CU as reported to the system. Inalternate embodiments, the location of the target CU may be derived inother ways.

Next, step 204 calculates a call opportunity area (e.g., callopportunity area 100, FIG. 4) for the target CU. The call opportunityarea is defined by the angle of elevation from the horizon that thesatellite is likely to be able to support communications with the targetCU. Step 206 then predicts a next satellite pass that might intersectthe call opportunity area. In a preferred embodiment, satellite passescan be predicted because a satellite's orbital parameters and the knowncurrent position can be extrapolated into the future to create apredicted future position of a satellite.

From the path of the next satellite pass, step 208 establishes anoptimal call initiation area (e.g., area 124, FIG. 5) as a portion ofthe call opportunity area. As described in conjunction with FIG. 5, theoptimal call initiation area is determined so that a call between thesource CU and the target CU may have a duration that is greater than aminimum acceptable call duration.

Step 210 determines whether a next satellite pass is within an optimalcall initiation area. If not, CF continues to predict satellite passesin step 206. If a next satellite pass does fall within an optimal callinitiation area, step 212 estimates the time that the satellite entersthe optimal call initiation area. In a preferred embodiment, CF alsoestimates the time the satellite exits the optimal call initiation area.In step 214, CF determines whether more future call initiationopportunities are desired. If so, CF repeats steps 206-214. If not, themethod ends in step 218.

FIG. 9 illustrates a communication unit 220 in accordance with apreferred embodiment of the present invention. CU 220 includes processor222, transceiver 224, antenna 226, and memory device 228. Transceiver224 receives messages via antenna 226 from satellites (e.g., satellite12, FIG. 1), including receiving future call opportunities. Processor222 is coupled to transceiver 224 and performs the functions of makingcall attempts and, in a preferred embodiment, providing a user with anoption whether to have the communication system calculate future callopportunities and dial-back the target unit. Memory device 228 iscoupled to processor 222 and is used for storing the future callopportunities, when they are provided.

In a preferred embodiment, CU 220 is provided with an option of whetherto have the future call opportunity sent to the CU 220, and CU 220further includes display device 230 and input device 232. Display device230 is coupled to processor 222 and is used to display the option to auser of CU 220. Input device 232 is coupled to processor 222 and is usedto allow the user to input information indicating that the user wants toexercise the option.

FIG. 10 illustrates a control facility in accordance with a preferredembodiment of the present invention. CF 240 includes processor 242,transceiver 246, antenna 248, and memory device 244. Transceiver 246 isused to communicate with satellites and CUs via antenna 248, includingsending future call opportunities to source CUs. Processor 242 iscoupled to transceiver 246 and is used for determining whether a sourceCU has successfully established a communication path with a target CU,calculating future call opportunities, contacting the target CU duringthe future call opportunities, and connecting the source CU and thetarget CU. Memory device 244 is coupled to processor 242 and is used forstoring information necessary to calculate future call opportunities.

In summary, a method and apparatus has been described for determining anoptimal call initiation opportunities and for connecting a source CU anda target CU during the opportunity. The foregoing description of thespecific embodiments will so fully reveal the general nature of theinvention that others can, by applying current knowledge, readily modifyor adapt for various applications such specific embodiments withoutdeparting from the generic concept and, therefore, such adaptations andmodifications should and are intended to be comprehended within themeaning and range of equivalents of the disclosed embodiments. Inparticular, while a preferred embodiment has been described in terms ofa satellite communicationsystem, the method and apparatus of the presentinvention may be utilized for any communication system having aliklihood of call success that varies due to predictable paths ofantennas.

It is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.Accordingly, the invention is intended to embrace all such alternatives,modifications, equivalents and variations as fall within the spirit andbroad scope of the appended claims.

What is claimed is:
 1. A method for establishing a communication path ina communication system having multiple communication units, at least onecommunication antenna that predictably changes a position with respectto a target communication unit, and at least one control facility, themethod comprising the steps of:a) determining whether a target link issuccessfully established when a source communication unit makes a firstcall attempt to establish the communication path with the targetcommunication unit, the communication path requiring establishment ofthe target link between the target communication unit and one of the atleast one communication antenna; b) where the target link is notsuccessfully established, calculating a future call opportunity when thetarget communication unit will have a better opportunity to support thetarget link because a predicted future position of one of the at leastone communication antenna will have moved within an optimal callinitiation area with respect to the target communication unit; c)attempting to contact the target communication unit during the futurecall opportunity; d) where attempting to contact the targetcommunication unit is successful, requesting that the targetcommunication unit hold while the source communication unit iscontacted; e) contacting the source communication unit; and f)connecting the source communication unit and the target communicationunit.
 2. The method as claimed in claim 1 further comprising the stepof:g) sending the future call opportunity to the source communicationunit so that the source communication unit has a knowledge of when thesource communication unit may be contacted during the future callopportunity.
 3. The method as claimed in claim 1 further comprising thesteps of:g) providing the source communication unit with an option ofwhether or not to have the communication system calculate the futurecall opportunity; h) when the source communication unit exercises theoption, performing steps b) through f; and i) when the sourcecommunication unit does not exercise the option, not performing steps b)through f).
 4. The method as claimed in claim 1 wherein the step ofcalculating the future call opportunity comprises the steps of:b1)establishing an optimal call initiation area within which one of the atleast one communication antenna should be located for an increasedlikelihood that the target link will be possible; b2) predicting a nextantenna to move within the optimal call initiation area; b3) estimatinga first time when the next antenna will enter the call initiation area;b4) calculating the future call opportunity as the first time; and b5)attempting to contact the target communication unit during the futurecall opportunity.
 5. The method as claimed in claim 4 wherein the stepof establishing the optimal call initiation area comprises the stepsof:b1a) determining a last-known location of the target communicationunit as a most recent location of the target communication unit reportedto the communication system; b1b) calculating a call opportunity areathat is defined by an angle of elevation between the last-known locationand a horizon such that one of the at least one communication antenna ismore likely to be able to support the target link; and b1c) determiningthe optimal call initiation area as a portion of the call opportunityarea, wherein the optimal call initiation area is determined so that acall between the source communication unit and the target communicationunit may have a duration that is greater than a minimum acceptable callduration.
 6. The method as claimed in claim 4 wherein the step ofcalculating the future call opportunity further comprises the stepsof:b6) determining a second time when the next antenna will exit theoptimal call initiation area; and b7) calculating the future callopportunity as times between the fast time and the second time.
 7. Themethod as claimed in claim 1 further comprising the steps of:g)calculating at least one additional future call opportunity; and h)where attempting to contact the target communication unit is notsuccessful, attempting to contact the target communication unit duringthe at least one additional future call opportunity.
 8. A method formaking a call connection in a communication system having multiplecommunication units, at least one satellite that predictably changes aposition with respect to a surface of an earth, and at least one controlfacility, the method comprising the steps of:a) making a fast callattempt by a source communication unit to establish a communication pathwith a target communication unit, the communication path requitingestablishment of a target link between the target communication unit anda satellite; b) determining whether the target link is successfullyestablished; c) where the target link is not successfully established,calculating a future call opportunity when the target communication unitwill have a better opportunity to support the target link because apredicted future position of one of the at least one satellite will havemoved within an optimal call initiation area with respect to the targetcommunication unit; d) attempting to contact the target communicationunit during the future call opportunity; e) where attempting to contactthe target communication unit is successful, requesting that the targetcommunication unit hold while the source communication unit iscontacted; f) contacting the source communication unit; and g)connecting the source communication unit and the target communicationunit.
 9. A method for establishing a communication path in acommunication system having multiple communication units, at least onecommunication antenna that predictably changes a position with respectto a target communication unit, and at least one control facility, themethod comprising the steps of:a) making a call attempt by a sourcecommunication unit to establish the communication path with a targetcommunication unit, the communication path including a target linkbetween the target communication unit and a target antenna that moveswith respect to the target communication unit; b) where the call attemptis not successful, waiting for the communication system to attempt tocontact the target communication unit during a future call opportunityduring which the target communication unit will have a betteropportunity to support the target link because a predicted futureposition of one of the at least one communication antenna will havemoved within an optimal call initiation area with respect to the targetcommunication unit; c) the source communication unit being contacted bythe communication system when the communication system successfullycontacts the target communication unit; and d) establishing thecommunication path with the target communication unit.
 10. The method asclaimed in claim 9 further comprising the steps of:e) the sourcecommunication unit being provided an option by the communication systemof whether or not to have the communication system contact the targetcommunication unit during the future call opportunity; f) presenting auser of the source communication unit with the option; g) when the userindicates that the user wants to exercise the option, performing stepsb) through d); and h) when the user indicates that the user does notwant to exercise the option, not performing steps b) through d).
 11. Themethod as claimed in claim 9 further comprising the step of:e) receivingfrom the communication system the future call opportunity so that thesource communication unit has a knowledge that the source communicationunit may be contacted during the future call opportunity.
 12. The methodas claimed in claim 9 wherein the step of waiting for the communicationsystem to attempt to contact the target communication unit comprises thesteps of:b1) establishing an optimal call initiation area within whichone of the at least one communication antenna should be located for anincreased likelihood that the target link will be possible; b2)predicting a next antenna to move within the optimal call initiationarea; b3) estimating a first time when the next antenna will enter theoptimal call initiation area; b4) calculating the future callopportunity as the first time; and b5) attempting to contact the targetcommunication unit during the future call opportunity.
 13. The method asclaimed in claim 12 wherein the step of establishing the optimal callinitiation area comprises the steps of:b1a) determining a last-knownlocation of the target communication unit as a most recent location ofthe target communication unit reported to the communication system; b1b)calculating a call opportunity area that is defined by an angle ofelevation between the last-known location and a horizon such that one ofthe at least one communication antenna is more likely to be able tosupport the target link; and b1c) determining the optimal callinitiation area as a portion of the call opportunity area, wherein theoptimal call initiation area is determined so that a call between thesource communication unit and the target communication unit may have aduration that is greater than a minimum acceptable call duration.
 14. Acommunication unit comprising:a transceiver for receiving messages fromat least one antenna that moves with respect to the communication unit;and a processor coupled to the transceiver for making a call attempt toestablish a communication path with a target communication unit, thecommunication path including a target link between the targetcommunication unit and a target antenna that moves with respect to thetarget communication unit, where the call attempt is not successful,waiting for a communication system to attempt to contact the targetcommunication unit during a future call opportunity during which thetarget communication unit will have a better opportunity to support thetarget link because a predicted future position of one of the at leastone antenna will have moved within an optimal call initiation area withrespect to the target communication unit, if the communication systemsuccessfully contacts the target communication unit, being called by thecommunication system, and establishing the communication path with thetarget communication unit.
 15. The communication unit as claimed inclaim 14 wherein the processor is also for being provided with an optionof whether or not to have the communication system contact the targetcommunication unit during the future call opportunity, the communicationunit further comprising:a display device coupled to the processor fordisplaying the option to a user of the communication unit; and an inputdevice coupled to the processor for allowing the user to inputinformation indicating that the user wants to exercise the option.
 16. Acall opportunity calculation apparatus comprising:a transceiver forsending messages to a source communication unit; a processor coupled tothe transceiver for determining whether the source communication unithas successfully established a communication path with a targetcommunication unit, the communication path including a target linkbetween the target communication unit and at least one antenna thatmoves with respect to the target communication unit, the processor alsofor calculating a future call opportunity at which the sourcecommunication unit will have a better opportunity to establish thetarget link because a predicted future position of one of the at leastone antenna will be within an optimal call initiation area with respectto the target communication unit, attempting to contact the targetcommunication unit during the future call opportunity, where attemptingto contact the target communication unit is successful, requesting thatthe target communication unit hold while the source communication unitis contacted, contacting the source communication unit, and connectingthe source communication unit and the target communication unit; and amemory device coupled to the processor for storing information necessaryto calculate the future call opportunity.